A site for medical students - Practical,Theory,Osce Notes

The vertebral artery

The vertebral artery arises from the following vessels
  Innominate artery on the right
 Subclavian artery on the left

     Segments of vertebral artery
Vertebral artery has four segments
The first (V1) -extends from its origin to its entrance into the sixth or fifth transverse vertebral foramen.
The second segment (V2) -traverses the vertebral foramina from C6 to C2.
·    The third segment (V3) -passes through the transverse foramen and circles around the arch of the  atlas to pierce the dura at the foramen magnum.
·   The fourth segment (V4) -segment courses upward to join the other vertebral artery to form the  basilar artery; Only the fourth segment gives rise to branches that perfuse the brainstem and cerebellum
      Course of verebral artery
Each vertebral artery passes upwards through the vertebral foramina to enter the cranial cavity through the foramen magnum and runs upwards on each side of the medulla. Both arteries meet at the lower border of the pons to form one midline single artery, the basilar artery, which runs upwards on the ventral surface of the pons were it gives small branches known as the paramedian arteries to the brain stem and divides into its two terminal branches the posterior cerebral arteries. Each posterior cerebral artery supplies the whole occipital lobe and the posterior part of the temporal lobe (posterior 2/5 of the cerebral hemisphere).
Branches of vertebral artery
In its course the vcrtebro-basilar system gives:
Two spinal arteries which unit to form the anterior spinal artery.
Three cerebellar arteries on each side. The superior middle and inferior cerebellar arteries.

Etiology of VBI
·       Atherothrombotic lesions have a predilection for V1 and V4 segments of the vertebral artery.
·  Atheromatous disease rarely narrows the second and third segments of the vertebral artery, thisregion is prone  to dissection, fibromuscular dysplasia, and, rarely, encroachment by osteophytic spurs situated within the vertebral foramina.
Clinical features of VBI
The first segment may become diseased at the origin of the vessel and it produce posterior circulation emboli; If there is sufficient  collateral flow from the contralateral vertebral artery or the ascending cervical, thyrocervical, or occipital arteries it is usually sufficient to prevent low-flow TIAs or stroke.
When one vertebral artery is atretic and an atherothrombotic lesion threatens the origin of the other, the collateral circulation, which may also include retrograde flow down the basilar artery, is often insufficient. This will promote, low-flow TIAs
This state also sets the stage for thrombosis.
Disease of the distal fourth segment of the vertebral artery can promote thrombus formation it will manifest as embolism or with propagation as basilar artery thrombosis.
Stenosis occurring proximal to the origin of the PICA can threaten the lateral medulla and posterior inferior surface of the cerebellum.
Embolic occlusion or thrombosis of a V4 segment causes ischemia of the lateral medulla.
What is “subclavian steal.”?
If the subclavian artery is occluded proximal to the origin of the vertebral artery this will result in reversal in the direction of blood flow in the ipsilateral vertebral artery. Exercise of the ipsilateral arm may result in  increase demand on vertebral flow, producing posterior circulation TIAs, or “subclavian steal.”
Clinical manifestations of Vertebrobasilar insufficiency
Clinicalfeatures of VBI consist of syncope, vertigo, and alternating hemiplegia
Hemiparesis is not a feature of vertebral artery occlusion, however, quadriparesis may result from occlusion of the anterior spinal artery.